This invention relates to a propfan engine and, more particularly, to a propfan engine having two oppositely rotating fan rotors; a gas turbine whose low-pressure turbine has two contra-rotating turbine rotors which drive the fan rotors by way of two coaxial turbine shafts, and a multi-stage low-pressure compressor, i.e., a booster, which has contra-rotating concentric booster rotors.
Previously known solutions of two-stage contra-rotating fan engines, particularly propfan engines, are constructed either with transmissions which transmit the whole power supplied by the low-pressure turbine and carry out the power distribution to the fan rotors, or each of the two fan stages is driven by a separate shaft.
The use of a low-pressure compressor, i.e., a booster, which is arranged upstream of the gas turbine in the primary circuit for increasing the thrust, does not present any problems in the case of transmission solutions because it may easily be mounted on the low-pressure turbine shaft. The transmission itself, which is usually constructed as a differential transmission--that is, the drive of the rearward fan stage takes place by way of ring gear, and that of the forward fan stage takes place by way of the sun wheel--represents a high technical challenge. In addition, this type of transmission requires high rotational speeds of the low-pressure turbines because the transmission ratio of such transmissions cannot be constructed to be significantly below i=4.0 to 4.5. According to the size of the fan diameter, this leads to rotational speeds of above 8,000 r.p.m. for the low-pressure turbine, which is feasible aerodynamically, but presents problems technically.
In addition to the use of a transmission, the direct drive of the two fan stages, by way of two shafts, is another possibility for solving the problem. In this case, a multi-rotor low pressure turbine is a prerequisite, the rotors of which are arranged either behind one another or are contra-rotating inside one another. In this case, the turbine, whose rotors are arranged behind one another, has a very long construction.
The most compact construction is a contra-rotating lower pressure turbine where the rotors are arranged inside one another, as disclosed in the German Patent Document DE-38 12 027 A1. The drive of the low-pressure compressor arranged between the fan rotors and the gas turbine, in this case, takes place by a fan rotor shaft by way of a transmission. The shafts are therefore loaded to a different extent. However, the additional transmission increases the weight and requires additional maintenance expenditures.
In contrast, in the German patent Document DE-37 28 436 A1, a gearless contra-rotating low pressure compressor is disclosed which is arranged between the two fan rotors. In this arrangement, however, there is no increase in protection of the second rotor against the effect of foreign bodies, such as encounters with birds, because the air inlet of the core engine must be arranged in front of the rearward fan rotor. Another disadvantage is the necessarily larger hub ratio of the rearward fan rotor.
Based on the above, there is needed a propfan engine of the above-mentioned type having a contra-rotating booster compressor driven by the low-pressure turbine without the use of a transmission, and the hub ratio of the fan rotors is not affected disadvantageously. A maximal protection of the primary circuit must be ensured against the effect of foreign bodies.
According to the invention, this need is met in that the low-pressure compressor is arranged coaxially between the fan rotors and the gas turbine. A radially interior booster rotor is connected with the radially interior turbine shaft. A radially exterior booster rotor is connected on the fan-rotor-side, by way of the row of blades of the first booster stage, with the fan rotor that is second in the flow direction. On the gas-turbine-side, the radially exterior booster rotor, by way of the row of blades of the last booster stage, is connected with the radially exterior low-pressure turbine shaft, and the air inlet of the gas turbine being arranged downstream of the fan rotors.
The arrangement according to the invention has the advantage of uniformly loading the driving turbine shafts by the arrangement of the contra-rotating booster rotors in the transmission line between the fan rotors, on the one side, and the gas turbine, on the other side, while the overall length of the booster is short. An optimal protection against the effect of foreign bodies on the core engine and on the booster is provided by the fact that both fan rotors are arranged upstream of the air inlet to the primary circuit and sucked-up foreign bodies are therefore deflected from the inlet by two fan rotors. In addition, the hub ratio in the area of the fan rotors may be kept small because the booster is arranged downstream of the rearward fan rotor. As a result, the required engine diameter is correspondingly reduced.
Another embodiment of the present invention permits a high compression ratio while the size of the booster is compact since the contra-rotating capacity of the booster rotors ensures high compression ratios also when the rotational speed is low.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.